I. Field of the Invention
The present invention relates to an X-ray diagnostic apparatus and, more particularly, to an apparatus capable of varying a high voltage, applied between an anode and a filament of an X-ray tube (hereafter referred to as "an X-ray tube voltage") in accordance with preset, stored data, in a short time to instantaneously produce photographed images of a plurality of subjects to be inspected (e.g., a stomach, a backbone or lungs).
II. Description of the Prior Art
According to radiation biographical experiments and theoretical researches, an X-ray absorption coefficient is different, as shown, for example, in FIG. 1, depending upon the tissue of a subject to be photographed. In FIG. 1, an ordinate axis is an X-ray absorption coefficient relative to water, and an abscissa axis is an X-ray wavelength. Curve 1A is an X-ray absorption characteristic of a bone, curve 1B is an X-ray absorption characteristic of water and muscles, and curve 1C is an X-ray absorption characteristic of fat. As seen from the characteristic diagram, the X-ray absorption difference of the bone, muscles and fat of the subject is large when the X-ray of long wavelength, generated by a low voltage, is permeated through the subject. However, the absorption difference of the bone, muscles and fat of the subject is small when the X-ray of short wavelength, generated by a high voltage, is permeated through the subjects. Thus, a difference of representing the photographed image occurs when the same object is photographed by the X-ray generated by high voltage and when the same object is photographed by the X-ray generated by low voltage, as shown in FIGS. 2(A) and 2(B). FIG. 2(A) shows the analyzed result of the X-ray photographed image using high voltage, and FIG. 2(B) shows the analyzed result of the X-ray photographed image using low voltage. Ordinate axis of FIGS. 2(A) and 2(B) are the blackness (density) of a photosensitive member (a photographing film), and abscissa axis are the X-ray permeation rate.
When the characteristics, X-ray photographic characteristics of objects to be diagnosed, are considered, the relationship between the tube voltage value for generating an X-ray and exposure time of a film is, for example, set as shown in FIG. 3, thereby providing a preferable photograph of the object to be diagnosed.
In FIG. 3, an ordinate axis is the X-ray tube voltage value, and an abscissa axis is an exposure time. When a subject to be diagnosed is the lungs (which are almost fat), as understood from the photographic characteristic of FIG. 2(A), the lungs of sufficient blackness are presented on the photograph due to the fact that a high X-ray tube voltage value is set in the apparatus. When a subject is a backbone (bone), the X-ray tube voltage value is low, and the exposure time is set to be long. As understood from the photographic characteristic of FIG. 2(B), this is a technique for presenting the bone of sufficient blackness and suppressing the blackness of other portions except the bone. When the object is a stomach (muscles), the X-ray tube voltage value is set substantially to the intermediate value of the X-ray tube voltage value at the time of photographing the lungs and at the time of photographing the backbone. This utilizes the fact that the voltage value is insufficient at the time of photographing the lungs, and the exposure time is insufficient at the time of photographing the backbone, with the result that the image of the stomach can be presented relatively clearly.
Therefore, in conventional X-ray equipment, the X-ray tube voltage value is varied in response to the object to be diagnosed, suitable X-ray tube voltage value is set to the subject to be diagnosed, and the X-ray photographing is then carried out.
When a plurality of pieces of diagnostic information of the same object to be photographed are desired, i.e., X-ray photographs of the optimum blackness (density) of the lungs, stomach or backbone of the same object to be diagnosed are desired, the photographs can be obtained by a first or second method by conventional X-ray equipment as will be described.
The first method includes photographing the object several times at different X-ray tube voltage values (which means irradiating X-rays to the object several times) to obtain a plurality of X-ray photographs. The second method includes photographing the object on one film several times at different X-ray tube voltage values.
However, according to the first method, there is a problem that the X-ray exposure amount of the subject increases in response to the number of photographings. Further, the number of films to be used increases, and it takes plenty of time to exchange the films. According to the second method, there arises a problem similar to the first method in that the X-ray exposure amount of the object increases. Further, since it takes plenty of time to stabilize the X-ray tube voltage to a desired value in the conventional X-ray equipment, the object moves, with the result that the X-ray image is blurred. The variation in the X-ray tube voltage value of the conventional X-ray equipment is obtained by the selection of taps of the primary winding of a transformer or the movement of a carbon roller. In both cases, a mechanical manual operation by an operator is necessary. Therefore, the photographing time takes long, and the possibility of blurring the image is high.